1. Field of the Invention
This invention relates to a ferroelectric liquid crystal composition and more particularly it relates to a ferroelectric liquid crystal material suitable for guest-host type display elements containing a dichroic dyestuff.
2. Description of the Related Art
At present, liquid crystal compounds have been broadly used for display materials, and most of such liquid crystal display elements rely on a TN (Twisted Nematic) display mode, and also liquid crystal materials used belong to a nematic phase. Display elements by way of this TN display mode have come to be rapidly used making use of specific features which do not tire the eyes. Due to their non-emissive type, their electric power consumption is very small and they are lightweight and of small size. Those having a large display area have recently been also prepared, but a drawback that the response is slow and the display cannot be seen depending on angle of view has not yet been overcome. In order to make use of the merits of liquid crystal display elements and supplement their shortcomings, development of a novel display mode in place of the TN display mode is indispensable. One of such attempts is a display element utilizing the light-switching phenomenon of ferroelectric liquid crystals proposed by N. A. Clark et al (see Applied Physics Letters, 36, 899 (1980)). The presence of ferroelectric liquid crystals has been elucidated by R. B. Meyer et al for the first time in 1975 (see J. de Phys., 36L, 69 (1975)), and ferroelectric properties are developed only in the case of smectic phases having a helical structure (chiral smectic phases). It has been known that chiral smectic phases include smectic C phases, I phases and F phases (hereinafter abbreviated to S.sub.C * phase, S.sub.I * phase and S.sub.F * phase).
In the chiral smectic phases, liquid crystals form layers, the molecules of the crystals tilt against the respective surfaces of the layers and the tilt direction deviates little by little from one of the layers to the succeeding ones to form a helical structure, the helical axis thereof being perpendicular to the surfaces of the layers. In the chiral smectic phases, spontaneous polarization is formed and hence when a direct current electric field in the direction parallel to the layers is impressed thereto, the molecules are inverted around the helical axis as a rotating axis depending on the polarity of the polarization. The display elements making use of the ferroelectric liquid crystals utilize this switching phenomenon.
Among the chiral smectic phases; the S.sub.C * phase has now been particularly noted. As the display mode utilizing the switching phenomenon of the S.sub.C * phase, two modes are considered. One mode is of a birefringent type using two sheets of polarizers and another mode is of a guest-host type utilizing dichroic dyestuffs.
The specific features of these display modes consist in that
(1) the response rate is very high; PA1 (2) there are memory properties; PA1 (3) the dependency on angle of view is small; etc.; hence the modes have the possibility of a high density display and are very attractive. In addition thereto, according to the display mode of the guest-host type, since only one sheet of polarizer is used, the mode has the following superior specific features: PA1 (4) there is no coloration due to retardation; and PA1 (5) the quantity of transmitted light is so large that the image is bright and readily visible. In particular, the item (4) means that it is unnecessary to precisely control the cell thickness of display elements; hence the mode is commercially very favorable. PA1 (1) the tilt angle is in the range of 30.degree. to 60.degree.; PA1 (2) the alignment properties are good; PA1 (3) the materials have an S.sub.C * phase which is stably operable at room temperature; PA1 (4) the spontaneous polarization is great; PA1 (5) the helical pitch is long; etc. PA1 (i) a mixture comprising at least one liquid crystal having a smectic A phase region within 40.degree. C. on the higher temperature side relative to a chiral smectic C phase and at least one liquid crystal having a chiral smectic C phase or achiral smectic C phase and having no smectic A phase region; PA1 (ii) a mixture comprising at least one liquid crystal having a smectic A phase region within 40.degree. C. on the higher temperature side relative to an achiral smectic C phase and at least one liquid crystal having a chiral smectic C phase and having no smectic A phase region; or PA1 (iii) a mixture comprising at least one liquid crystal having smectic A phase region within 40.degree. C. on the higher temperature side relative to achiral smectic C phase, at least one liquid crystal having achiral smectic C phase and having no smectic A phase region and at least one optically active substance.
The main conditions required for ferroelectric liquid crystal materials used for display elements of guest-host type are as follows:
Particularly, the conditions of the tilt angle and alignment property are indispensable because display elements which do not satisfy these conditions bring about notable reduction in the contrast.
The range of the above tilt angle is calculated as follows:
The absorption intensity is calculated by means of dyestuffs, of display elements of the guest-host type having a tilt angle of .theta. is proportional to sin.sup.2 (2.theta.) and maximal at .theta.=45.degree.. In this case, even 75% of the maximum absorption intensity does not differ so much practically from that at .theta.=45.degree. in the aspect of contrast; hence the range of .theta. affording 75% of the maximum absorption intensity, that is, the limit of .theta. which is tolerable in the aspect of contrast, can be calculated as being in the range of 30.degree. to 60.degree..
Further, with regard to the uniformity of alignment, the following has been known:
When the alignment of liquid crystal molecules is non-uniform, that is, the so-called multi-domain state is formed, unevenness is formed in the resulting display so that the contrast ratio is reduced or the display surface is colored, which results in a notable reduction in the display quality. There is an intimate relation between the alignment property and the kinds of phases existent on the higher temperature side relative to S.sub.C * phase. In short, in the case of liquid crystals having a smectic A phase (abbreviated to S.sub.A phase) on the higher temperature side relative the S.sub.C * phase, since the normal direction to the layer surface accords with the direction of the major axis of liquid crystal molecule, a relatively good aligned state is obtained, whereas in the case of liquid crystals having no S.sub.A phase on the higher temperature side relative to an S.sub.C * phase, a cholesteric phase (abbreviated to Ch phase) or an isotropic liquid phase (abbreviated to I phase), during the cooling process of the material, transits directly into the S.sub.C * phase having two stable energetically degenerated directions along which the major axis of the liquid crystal molecules may be oriented. Thus, without breaking this energetic degeneracy by impressing an electric field or the like thereto at the time of alignment, a good aligned state is not obtained; hence the material is not practical. In short, in the aspect of the alignment property, the compounds having an S.sub.A phase on the higher temperature side relative to the S.sub.C * phase are far superior to those having no S.sub.A phase on the higher temperature side relative to the S.sub.C * phase (for example, see Japanese patent application laid-open No. Sho 61-255323).
Currently known ferroelectric liquid crystal compounds are illustrated in Table 1. The reports and the literature describing these compounds are found in PH. Martinot-Lagarde et al., Mol. Cryst. Liq. Cryst., 75, 249 (1981); Inukai et al, collected preprints for the 10th Liquid Crystal Symposium, pp 164, pp 166 (1984); Kayako Hori, collected preprints for the 10th Liquid Crystal Symposium, pp 112 (1984); Inukai et al, collected preprints for the 11th International Liquid Crystal Conference, .theta.-018-FE (1986); etc.
As apparent from Table 1, among currently known ferroelectric liquid crystal compounds, most of the compounds having tilt angles of 30.degree. or more at their S.sub.C * phase are compounds having no S.sub.A phase on the higher temperature side relative to S.sub.C * phase, while most of compounds having an S.sub.A phase thereon are compounds having a tilt angle of 26.degree. or less. Thus it has been considered that there is an intimate relation between the phase series of liquid crystal compounds and the tilt angle thereof (for example, see Tetsuro Ohtsuka, collected preprints for the 12th Liquid Crystal Symposium, pp 98 (1986)).
Namely, compounds having an S.sub.A phase on the higher temperature side relative to the S.sub.C * phase are superior in the alignment property, but since they have a tilt angle of 26.degree. or less, they are inferior in the aspect of contrast, while compounds having no S.sub.A phase thereon are inferior in the aspect of the alignment property. Thus it is impossible to use the existing compounds, as they are, as materials for display elements of the guest-host type. TBL3 TABLE 1 Phase transition Tilt angle (.degree.) Compound point T-T.sub.c = -10.degree. C. T-T.sub.c = -20.degree. C. ##STR1## ##STR2## 23 ##STR3## ##STR4## 22 ##STR5## ##STR6## 19.8 ##STR7## ##STR8## 15.6 18.6 ##STR9## ##STR10## 15.1 20.3 ##STR11## ##STR12## 16.3 21.2 ##STR13## ##STR14## 44 45 ##STR15## ##STR16## 32 34 ##STR17## ##STR18## 45 45 In Table 1, the tilt angle refers to values measured at a temperature lower by 10.degree. C. or 20.degree. C. than the upper limit temperature of S.sub.C * phase (expressed by T-T.sub.c = -10.degree. C. or T-T.sub.c -20.degree. C., respectively). In the column of the phase transition point, C, S.sub.B and S.sub.H * mean crystalline phase, smectic B phase and chiral smectic H phase, respectively.